Re: [PATCH] x86/syscalls: Mark expected switch fall-throughs

[Gustavo A. R. Silva]

Quoting Thomas Gleixner <tglx@xxxxxxxxxxxxx>:

> On Tue, 28 Nov 2017, Thomas Gleixner wrote:
>
> +CC Linus.
>
> > On Tue, 28 Nov 2017, Thomas Gleixner wrote:
> >
> > > On Tue, 28 Nov 2017, Gustavo A. R. Silva wrote:
> > > > Quoting Thomas Gleixner <tglx@xxxxxxxxxxxxx>:
> > > > > > To be honest, such comments annoy me during a code review  
> > especially when
> > > > > > the fallthrough is so obvious as in this case. There might  
> > be cases where
> > > > > > its worth to document because it's non obvious, but documenting the
> > > > > > obvious
> > > > > > just for the sake of documenting it is just wrong.
> > > > >
> > > >
> > > > I understand that and I agree that in this particular case it  
> > is just obvious.
> > > > The thing is that if we want to benefit from having the  
> > compiler help us to
> > > > spot these kind of issues before committing our code, we have  
> > to address every
> > > > place in the whole code-base.
> > > >
> > > > > And _IF_ at all then you want a fixed macro for this and not a comment
> > > > > which will be formatted as people see it fit.
> > > > >
> > > > > GCC supports: __attribute__ ((fallthrough)) which we can wrap  
> > into a macro,
> > > > > e.g. falltrough()
> > > > >
> > > > > That'd be useful, but adding all these comments and then  
> > having to chase a
> > > > > gazillion of warning instances to figure out whether there is  
> > a comment or
> > > > > not is just backwards.
> > > > >
> > > >
> > > > I have run into this before and people find what you suggest  
> > even uglier.
> > >
> > > It's not about ugly. It's about _USEFULL_.
> > >
> > > The comments are ugly AND completely useless for the compiler and they are
> > > going to be malformatted so checker tools can't differentiate the false
> > > positives.
> > >
> > > The macro, in which more or less ugly form written, is both documentation
> > > and helps the compiler NOT to emit the same crap over and over.
> >
> > Just checked and GCC really supports analyzing the comment to some extent.
> >
> > But just look at
> >
> >     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77817
> >
> >  " It is not really possible.  __attribute__((fallthrough)) has precise
> >    rules on where it can appear, while /* FALLTHRU */ comments, being
> >    comments, can appear anywhere.  Especially with -Wimplicit-fallthrough=1
> >    when all comments are considered fallthru comments... "

This is what we want to add:

# Warn about missing switch break or fall-through comment.
KBUILD_CFLAGS  += $(call cc-option,-Wimplicit-fallthrough)

> > I have no idea who came up with that brilliant idea of parsing comments in
> > the code. It's so simple to make this parser completely fail that it's not
> > even funny anymore.

I don't get why someone would want to do that to himself. :/

> > I don't care what other people prefer. The code base I'm responsible for
> > gets either proper annotations or nothing.
>
> And in fact we want ONE solution for the whole kernel. And comments are
> obviously the wrong one.

OK. I'll discuss this and see how we can come up with the best solution.

Thank you for your feedback
--
Gustavo A. R. Silva